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   GRE Permutation Guide
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spacer left_arrow Ch 1: Permutation Introduction
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spacer left_arrow Ch 2: Problem Variations
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   GRE Combination Guide
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spacer left_arrow Ch 1: What is a Combination?
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spacer left_arrow Ch 2: Permutation / Combination?
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spacer left_arrow Ch 3: Groups/Pairings
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   GRE Probability Guide
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spacer left_arrow Ch 1: Simple Probability
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spacer left_arrow Ch 2: Multiple Events
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spacer active_arrow Ch 3: Independent Events
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spacer left_arrow Ch 4: Working Backwards
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spacer left_arrow Ch 5: A Different Method
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spacer left_arrow Ch 6: Extra Questions
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   GRE Prep Course
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Chapter 3: Independent and Dependent Events
 
 

Whenever two or more events happen at the same time, i.e., when we use the word “and,” you will have to decide if the events are independent or dependent.

Examples 6 and 7 are examples of Independent probabilities. The outcome of the first event does not affect the probability of the second. Coin tosses are independent. They cannot affect each other's probabilities; the probability of each toss is independent of a previous toss and will always be 1/2. Separate drawings from a deck of cards are independent events ONLY if you put the cards back.

Dependent events are the opposite. The probability of the second event is affected by the first event. An example of a dependent event is drawing a card from a deck but not returning it. By not returning the card, you've decreased the number of cards in the deck by 1, and you've decreased the number of whatever kind of card you drew. If you draw an ace of spades, then there are 1 fewer aces and 1 fewer spades.


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 Chapter 4: Working Backwards